Hybrid electric vehicles may include a generator that may be used to control the speed of an engine. In response to a wheel power demand, a controller determines a corresponding engine power and electric motor power with the engine power used to select a target engine speed and torque. The target engine speed and torque may be selected based on various ambient and operating conditions using a calibratable table or map, which may be referred to as the energy management system (EMS) map. The EMS map is calibrated to achieve desired fuel economy based on various other considerations, including performance, emissions, and vehicle NVH, for example. The EMS map is generally calibrated based on sea level engine performance.
Some environmental and ambient operating conditions, such as higher altitudes (lower barometric pressure), higher ambient temperatures, or operating with an engine or component anomaly (such as a fouled spark plug, dirty fuel injector, restricted airflow, etc.), the torque available or attainable by the engine may be reduced at some or all engine speeds relative to the expected torque associated with an engine speed, temperature, etc. as contained in the EMS map. At such conditions, the engine may be incapable of supplying the torque specified by the EMS map at the target speed and the engine will not deliver the requested power at the EMS target speed. The vehicle system controller may then increase the engine speed to an operating point where the engine may deliver sufficient torque to satisfy the power demand.
For moderate accelerator pedal operation, the altitude compensated engine speed may be much higher than the engine speed would be at sea level and nominal ambient temperatures for the same power demand. The NVH associated with these high engine speeds may be objectionable to some customers. Furthermore, small changes in accelerator pedal position and associated power demand may result in large changes in engine speed without a proportional change in vehicle acceleration.